I think there are two distinct things to discuss here.
One is B/X as a whole. I think the reason it has resonated with OSR/retro aficionados so much is that it distills TSR D&D to its most essential parts. It's exploration-focused, combat is quick and straightforward, and characters are simple, without much in the way of balance issues. Magic is evocative and effective, without being overpowering, since the highest level spells are 6th level. It has a beginning, when the PCs are weak and vulnerable, a middle where the PCs are stronger and more capable, and an end where the PCs have established their own strongholds and followers. I agree with others in the thread that have suggested that much of AD&D play in the 80s was essentially B/X with AD&D races, classes, and monsters. B/X lends itself to being built upon, and expanded upon.
The other is Moldvay's basic rules as an introduction to the game. This is what I wrote at the end of my Let's Read of those rules:
The genius of Moldvay Basic lies in distillation. The "design" itself is still essentially OD&D/Holmes Basic Rules. There's not much innovative when looked at from that point of view. But Moldvay had a keen eye for cruft, and he cut away so much that, while interesting, was not needed. The Order of Events in One Game Turn, the Combat Sequence, I'm sure they seem passe when viewed from the perspective of today. But for an 11 year old, learning how to play and run the game just from reading the book, they broke everything into easily manageable chunks. While the effects of the sensational Egbert case cannot be discounted, I do think this is much of the reason B/X sold so well at that time.
The scope of D&D has expanded. It's not all about dungeon or wilderness exploration. It is not particularly about exploration at all. Combat, instead of being something quickly resolved, is now a load bearing pillar of the game, with much design thought put into character abilities and the balance thereof. The goal of adventure design is now in providing players with multiple choices through a storyline, ideally one tied to the backstories of the PCs, giving them personal stakes in what happens. This is not a criticism; this is the result of demand by the playerbase. That said, it's a lot for a first time DM to take on, least of all a first time DM who's never played before.
Moldvay guides the first time DM's hand by limiting the environment: it's a dungeon. The players' major choices are forward, backward, left, right, enter a room, or keep moving. He gives simple, concrete procedures: this is what you do on an exploration turn. This is what you do when there's an encounter. This is what you do when there's a combat. This is how you stock a dungeon. How many of us got our start with just that one 64-page book? And he wisely provides some room for PCs to grow. And with that very basic experience, the DM can build out: wilderness exploration, city exploration, heck, non-exploration adventures. The Mentzer Red Box and the Brown/Denning Black Box iterated on what Moldvay accomplished, but I'm not sure he's ever been bettered.